Method and apparatus for determining a relative distance in a cylinder and piston assembly

ABSTRACT

In the method and installation for determining a relative distance between the front wall (22) of the cylinder and the piston (14), an ultrasonic converter (30) is used to which a plate (68) presenting a reflecting reference surface (66) for the formation of a reference path is secured. The group of reference oscillations (R) and the group of measurement oscillations (M) appearing in the output signal of the converter (30) after the emission of a group of transmitter oscillations (S) are detected by means of threshold switches (108, 120, 122). Thereby, a measurement signal and a reference signal are obtained, the latter being used for compensation by means of a processor (38) for the influence of the different propagation conditions affecting the measurement signal.

FIELD OF THE INVENTION

The invention comprises a method for determining the relative distancebetween a piston and cylinder in a piston and cylinder assembly which isprovided with a chamber that is filled with a flowable medium, the wallsof the cylinder and the piston forming the chamber. The inventionfurther comprises the apparatus for accomplishing a method of this type.

BACKGROUND OF THE INVENTION

Methods and apparatuses of the above-referenced type are well known. Forexample, a measuring rod extending through the cylinder head wall ismoved with the piston, and the position of the rod is detectedcapacitively or resistively. It is also possible to drive anelectro-optical sensing device from the piston rod through atransmission device. Such a method requires large free space andsubstantially structural expense. Other well known distance measuringmethods are unsuited for the determination of the relative distancebetween the piston and cylinder head of the assembly because thenecessary measuring devices are too sensitive. Excluded equipment, forexample, includes forklifts with hydraulically operated lifting forks,earth-moving equipment with hydraulically adjustable shovels,hydraulically operated extrusion presses, and many other applications ofpiston and cylinder assemblies for which there is no satisfactory deviceto exactly determine the relative positioning.

SUMMARY OF THE INVENTION

It is a principal object of the present invention to provide a methodwhich constitutes an improvement over the prior art mentioned above,which can be accomplished at minimum expense and which delivers precisemeasuring results. It is a further object to provide a correspondingapparatus that is built with a minimum size at minimum expense and whichoperates precisely and reliably.

These objects are achieved according to the present invention in amethod of the type described above in which a group of sequentialultrasonic oscillations are transmitted from a first end wall of thecylinder chamber and after reflection in the region of the oppositelydisposed second end wall, the group received as a group of measurementoscillations. The elapsed time of the measurement oscillations betweenthe transmission of the group of ultrasonic oscillations and the receiptof the group of measurement oscillations is utilized as a measure of therelative distance. A fluid medium extends along a reference path of aprescribed length and at the first end of the path, the group ofultrasonic oscillations are transmitted and after reflection at areflecting reference surface forming the second end of the referencepath, the group of oscillations are received again as referenceoscillations. Through the measurement of the actual elapsed travel timebetween the transmission of the group of ultrasonic oscillations fromthe first end and the receipt of the reference oscillations, aproportional reference signal is produced, and the measurement signalobtained on the basis of the elapsed time measurement of the group ofmeasurement oscillations is modified in response to the reference signalin the sense of a compensation for the influence of changes in theultrasonic propagation conditions affecting the measuring signal.

Another object of the invention is to provide an apparatus foraccomplishing the method. Embodiments of the method and correspondinglythe apparatus are given in the following specification and claims.

According to the method and the apparatus of the present invention, agroup or packet of sequential ultrasonic oscillations are transmittedfrom one end of a cylinder chamber, are reflected at the oppositelydisposed end, and then are captured again at the first end. The traveltime is a relatively accurate measure of the distance between theoppositely disposed ends, and accordingly, the distance between a pistonand cylinder. Since the travel time for the ultrasonic oscillations inthe cylinder chamber is always dependent upon the propagation speed ofthe ultrasonic waves in the fluid medium filling the cylinder chamber,and the time changes with temperature, pressure, and the chemicalcomposition of the medium, specific measuring errors can accordingly becompensated for, whereby a reference signal is produced by means of areference path and the signal is combined with the measurement signalproduced by the elapsed time of the group of measuring oscillations as acorrection. The only device necessary for transmission and reception ofa group of ultrasonic oscillations for measurement of the travel time ofthe group of measuring oscillations, and if desired, also thesimultaneous measurement of the actual reference travel time is a simpleconverter which does not enlarge the size of the piston cylinderassembly, and the required electrical switching components for excitingthe converter and detecting the output signal from it. Such componentsand converter are relatively inexpensive and can easily be mounted at asuitable position which will not lead to a structural enlargement of themachine or, for example, the hydraulic system, in which the piston andcylinder assembly is employed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described below with the aid of the followingdrawings in which the an exemplary embodiment is illustrated.

FIG. 1 shows a piston and cylinder assembly and distance measuringapparatus according to the present invention. The piston and cylinderassembly and the converter of the distance measuring apparatus are shownin perspective, the remaining parts of the distance measuring apparatusin contrast being illustrated schematically in block diagram.

FIG. 2 is a side elevation view in perspective and shows the converterof the distance measuring apparatus in FIG. 1.

FIG. 3 is a somewhat detailed block diagram of the detection circuitryof the distance measuring apparatus in FIG. 1.

FIG. 4 is a timing diagram for explaining the operation of the detectioncircuitry in FIG. 3.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a piston and cylinder assembly 10 of a hydraulicsystem with the valves and manifolding omitted for simplicity. Theassembly includes a cylinder 12, a longitudinally reciprocated piston 14and a piston rod 18 extending through an end wall of the cylinder 12. Afurther end wall 22 of the cylinder 12 forms at its inner side the firstend 24 of the cylinder chamber 26 opposite the piston rod, the secondend 28 of which is located on the piston 14.

By way of example, for controlling the position of the tool driven bythe assembly 10 or a machine part or the output speed of a quantity ofmaterial from a pressure chamber having a pressure piston driven bymeans of the assembly 10, the relative distance between the cylinder 12and the piston 14 is continuously determined as the actual distance andthen, through subtraction of known values or time differentiation toprovide an actual speed value and so forth, additional desired valuescan be derived. For determining the relative distance between the piston14 and cylinder 12, an electrical ultrasonic generator or converter 30is positioned in the end wall 22 or in other possible embodiments, atleast in the vicinity of the first end face 24. The converter 30 isconnected by means of a cable 32 with a detection circuitry 34. Thelatter is comprised in one embodiment by an input/output device 36, aprocessor 38 and a data output device 40, which may be an indicatingdevice calibrated in units of length or a printer.

The converter 30, further illustrated in FIG. 2, is formed andpositioned so that it generates a group of ultrasonic oscillations inthe direction of the second end wall 28 when stimulated at least byelectrical impulses, and upon impacting of ultrasonic oscillations, theconverter produces a corresponding electrical output signal. Theconverter 30 has essentially a cylindrical shape forming a flatultrasonic reflective surface 42 with the end wall of the cylinderassociated with second end wall 28 of the chamber 26 which is in contactwith the flowable medium in the cylinder chamber 26. A carrier 44 isformed from an incompressible material at the end of the converter 30disposed oppositely from the ultrasonic reflective surface 42, and in apreferred embodiment, the carrier is composed of steel. The carrier 44has a flat surface parallel to the beam reflecting surface 42, and inFIG. 2, the circumferential edge 46 of the surface is recognizable.Between the ultrasonic reflective surface 42 and the surface 46 lies aplate-shaped piezoelectric crystal 48 which is parallel to and spacedfrom each of the surfaces. The transducer 48 is comprised of a suitableceramic material which is provided on each side with electrodes that arenot visible. The thickness of the transducer is precisely a halfwavelength of the ultrasonic oscillations and the ceramic material whichhas been selected for use, whereby the transducer establishes thefrequency of the ultrasonic oscillations by which groups of sequentialultrasonic oscillations are transmitted from the converter 30. Thisfrequency in one preferred embodiment is approximately 500 kHz. Thediameter of the transducer 48 and the like diameter of the ultrasonicreflective surface 42 must be a multiple of the ultrasonic wavelength ofsuch oscillations transmitted in the flowable medium; in the preferredembodiment, the referenced diameter is 20 millimeters.

In order for the oscillations 48 to be transformed into the greatestpossible ultrasonic energy through energy received from electricalexcitation pulses, and to be received by the converter 30 as thestrongest possible reflected oscillations after reflection at the secondend 28, and to be again transformed into an output signal, errorcompensation means are provided for wave resistance between thetransducer 48 and the carrier 44, as well as means for wave resistancecompensation between the transducer 48 and the ultrasonic reflectivesurface 42. Error compensation comes about essentially by intermixinglayers which have strongly differentiated wave resistances relative toone another. The ultrasonic wave resistance of a layer dependsessentially on the compressibility of the material and on the ultrasonicspeed in the material. The wave resistance of the transducer 48 and thecarrier 44 is relatively high, and for error compensation in a preferredembodiment, a quarter wavelength plate 50 is inserted between the twoand made of a relatively incompressible material, such as an epoxyresin. As a quarter wave plate, a plate is illustrated in which thethickness comprises a quarter wavelength of the ultrasonic waves in thematerial of the plate at the described frequency of the transducer 48.If desired, additional plates can be provided between the transducer 48and the carrier 44 with intermixed high and low wave resistances, forexample, a quarter wave plate 52 made from steel and a further quarterwave plate 54 made from an epoxy resin, which the dotted lines indicate;the exposed upper surface of the carrier 44 associated with thetransducer 48 would then lie adjacent the plate 54.

A wave resistance compensation means between the transducer 48 and thefluid medium in contact with the reflective surface 42 effectivelyproduces a lessening of the wave resistance in the space between thetransducer 48 and the reflective surface 42, and moves the resistanceaway from that of the transducer toward that of the compressible medium.Assuming that the fluid medium is an ordinary hydraulic fluid, the waveresistance of the epoxy resin is approximately twice that of thehydraulic fluid and the wave resistance of the common ceramic materialsused in the oscillator 48 is approximately 20 times that of the fluid.For wave resistance compensation, more quarter wave plates are providedin the preferred embodiment; four plates 56, 58, 60, 62 lie next to oneanother relative to the oscillator 48, and they have a type ofresistance selected so that they decrease in a manner corresponding to astepped geometric sequence from the transducer 48 to the hydraulicfluid. The plate 62 is comprised of glass with a wave resistanceapproximately 15 times higher than that of the hydraulic fluid. Theplate 60 is comprised of glass with a wave resistance approximatelyseven times higher than the hydraulic fluid.

The plate 58 is cut out of an ordinary circuit board that is comprisedof glass fiber material with epoxy resin and has a wave resistanceapproximately four times higher than the hydraulic fluid. Finally, theplate 56 forming the ultrasonic reflective surface 42, as well as theplate 50, is composed from epoxy resin with a wave resistanceapproximately twice as high as the compressibility. All the plates, 56,58, 60, 62 and possibly 52, 54, the transducer 48, and the cylindricalarea of the carrier 44 adjoining the plates 50 or correspondingly theplate 54 are provided generally, in a manner not illustrated, with anepoxy resin coating which serves as a mechanical connector for theseparts on their outer surfaces. At the same time, a connection of thetransducer 48 and the plates 50, 56, 58, 60, 62 and if included 52 and54, is produced by means of a thin adhesive coating that is notillustrated at their interconnecting adjacent surfaces.

By means of the converter 30, a group of successive ultrasonicoscillations are transmitted, controlled and partially detected throughan input/output device 36 (FIG. 1). These ultrasonic oscillations extendfor the most part to the second end wall 28 of the cylinder chamber 26and there are reflected as indicated by means of the arrow 64 in FIG. 1.Insofar as the second end wall 28 is not even and does not extendperpendicular to the axis of the cylinder 12, a retro-reflector, acentral tapered milled cut in the piston 14 with anequilateral-rectangular cross-section or an annular groove formed in thesecond end wall 28 with an equilateral-rectangular cross-section can beprovided to permit the ultrasonic oscillations reflected in the regionof the second wall 28 to be sent directly back in the direction of theconverter 30. The transmitted group of ultrasonic vibrations travel backto the converter 30 as a measurement group with an elapsed travel timeof the oscillations that is linearly proportional to the distancebetween the converter and the second wall 28. The group of reflectedoscillations are received at the converter and are transformed into agroup of measurement oscillations in the electrical output signal of theconverter 30 and by means of the processor 38 serve the purpose ofgenerating a measurement signal that is proportional to the relativedistance being sought between the cylinder 12 and the piston 14. Thesignal can be displayed on the data output device 40.

Variations in the ultrasonic propagation conditions in the cylinderchamber 26 caused for example by variations in temperature or pressureof the fluid medium can produce errors in the received measuring signal.For compensation of certain variations of the measurement signal, thefluid medium is disposed along a reference path of prescribed length, atthe first end of which a group of ultrasonic oscillations aretransmitted and after reflection at a reference reflecting surfaceformed at the second end of the reference path, they are again receivedas a group of reference oscillations. By measuring the actual traveltime elapsed between the transmission of the group of ultrasonicvibrations from the first end and the receiving of the reference groupof oscillations, the reference signal proportional to such time isproduced. The desired compensated signal is produced on the basis of thereference signal. If, for example, the measurement of the actual traveltime amounts to 1.1 times the predetermined travel time of a temperaturestablized quartz oscillator, it can be taken that the travel time of thegroup of oscillations associated with the measurement signal amounts to1.1 times the correct travel time which corresponds to the real oractual relative distance. Wherefore the value of the receivedmeasurement signal must be divided by a factor of 1.1 in order to obtaina corrected measurement signal precisely proportional to the relativedistance. This calculation of the compensation takes place in theprocessor 38.

Basically, the reference path can be located outside of the cylinderchamber 26 and remotely of the limited measurement path of the converteras long as it is guaranteed that the conditions along the reference pathwith regard to temperature, pressure, and physical-chemical compositionof the fluid mediums also prevail in the cylinder chamber 26. So, forexample, it may be useful to duct the hydraulic fluid from a pluralityof parallel working piston and cylinders assembly through a commonreference cylinder in which the reference path is formed in essentiallythe same manner as has been already described with the aid of FIG. 1 forthe measuring path. This reference path comprises therefore a specialconverter for transmitting and receiving ultrasonic oscillations. Formost applications, it is best with regard to both the construction costsand achievable accuracy of the desired compensation if the measuringpath and the reference path are the same, as in the disclosed embodimentof the converter, because in the cylinder chamber 26, a fixed referencereflection surface 66 is provided at a minimal distance from theconverter 30. The reference reflection surface 66 in the describedembodiment of the converter is formed at the back side of a steel plateassociated with the converter 30. The plate at its edge is secured tothe converter 30 by means of three wires that form bows 70. The bows 70project into the wave reflecting surface 42 near its outercircumference, penetrate the plate 56, and are soldered and held insolder holes 72 in the plate 58 as is indicated in FIG. 2. From themanufacturing standpoint, it is preferable that the plate 58 be cut froma material comprised of ordinary conductive platinum to produce adesired wave resistance. The solder holes or eyelets 72 can be formed inthe customary simple manner and are then soldered. After soldering thebows 70, the plate 56 with its beam reflecting surface 42 can then beformed in a simple manner by pouring an epoxy resin over the ordinarycomponent parts of the converter 30.

The distance of the reference reflection surface 66 from the beamreflection surface 42 of the converter 30 is in the preferred embodiment40 millimeters, and at that value, is smaller than the smallest variabledistance between the second end wall 28 and the first end wall 24 of thecylinder chamber. This dimension is established in order to notunnecessarily shrink or reduce the stroke of the piston 14 within thecylinder 12 on the one hand, and on the other hand, to permit firstlydetection of a group of reference oscillations from the converter 30after transmission of a group of ultrasonic oscillations and thereafterreceipt of the group of measurement operations with cleardifferentiation, the group of measurement oscillations appearing intimed relationship with one another in the output signal oftheconverter. Further, the reference reflection surface 66 is essentiallysmaller than the beam reflecting surface 42. In the preferredembodiment, the relationship of the surfaces in three to four percent.In this manner, it is guaranteed that the largest part of the projectedultrasonic energy reaches the second end wall 28 of the cylinder chamber26 and after reflection, gets back to the converter 30. Together withthe properties established due to the construction of the converter 30,the electrical excitation pulses propagating without energy loss in theultrasonic oscillations and reflected oscillations transformed intoelectrical output signals make it possible to receive groups ofreference oscillations that are sufficiently large and suitable forprecise compensation on the one hand, and on the other hand, make itpossible to measure with great accuracy distances having magnitudes ofup to 600 millimeters between the converter 30 and the second end wall28.

In order that a group of reference oscillations reflected from thereference reflection surface 66 do not bounce back and forth again inthe reference path after a signal or only insignificant resultingreflections at the beam reflection surface 42, that is, to avoiddisturbing reference echos, it is useful to arrange the referencereflection surface 66 opposite a position parallel to the beamreflection surface 42 with slight tilt. In a preferred embodiment, theperpendicular to the reference reflection surface 66 assumes an angle oftwo degrees to the longitudinal axis of the cylinder 12 and is locatedconcentrically with the axis of the converter 30.

The construction of the threshold circuit 34 (FIG. 1) is furtherillustrated in FIG. 3. The processor 38 operates in dialogue withinput/output device 36 through a connecting cable 74. The connectingcable 74 comprises a conductor 76 for the transmission of control bitsfrom the processor 38 to a data coupler 78 of the input/output device36, a conductor 80 for the transmission of eight data bits from the datacoupler 78 to the processor 38, and a conductor 82 for the transmissionof eight address bits between the processor 38 and data coupler 78 andback again. In regard to the measuring process taking place in theinput/output device, the processor 38 has a special control function andan evaluating and in particular a calculating function while theprogrammed processing of the required individual instructions formeasuring the travel time within the control results from a control unit84 and a buffering of the data contained therein in a buffer member 86within the input/output device 36. The actual data from the buffermemory together with a timing signal derived from the clock 88 areretrieved according to the commands from the buffer memory 86 and aretransferred through the coupler 78 to the processor 38 in order there tocompute the actual reference travel time, or the measurement oscillationtravel time and a correction measurement signal whereby theretransmissions to the processor 38 follow as permitted by the control dueto an interrupt circuit 90. In this manner, for example, thetransmission of a group of ultrasonic oscillations and the determinationof the timing position of the group of measurement oscillations and thegroup of reference oscillations relative to the transmission is produced250 times a second and the stored data obtained thereby can accordinglybe transferred only one per second or up to 250 times a second to theprocessor 38 for evaluation to establish if the piston is stopped ormoving and correspondingly causes no changes in the data or fast changesin the data.

The input/output device 36 comprises a sending unit 92 for producing aregulating pulse for four ultrasonic converters on conductors 32A-32D.In this manner, it is possible to carry out the measurement of therelative position of four piston and cylinder assemblies of the typeillustrated in FIG. 1. The sending unit 92 comprises an electrical pulsegenerator 94 that actually produces two square wave pulses that areseparated from one another for regulating a group of transmittedultrasonic oscillations. The sending unit also has a multiplexer 96connected to the pulse generator 94, and the outputs of the multiplexerlead to choke coils 100A-100D connecting downstream with the conductors32A-32D. The coils 100A-100D actually form a resonant circuit with theassociated converters 30 (FIGS. 1, 2) connected with the conductors32A-32D. The resonant circuit is tuned to a desired frequency of theultrasonic oscillations from the converter 30 in the preferredembodiment, for example, 500 Khz. The length and the timing interval ofthe square wave pulses produced by the pulse generator 94 are so tunedto the resonant frequency of said resonant circuit that these pulsesfrom the regulator can be controlled with the strongest manner possible.The multiplexer 96 causes a switching of the conductors 32A-32D in acyclic sequence.

To detect the output signals of the converter connected to theconductors 32A to 32D the conductors are also connected to a receiver102 and in particular to the inputs of a multiplexer 104 providedtherein. The multiplexers 96, 104 are controlled from a control register106 provided in the controller 84, such that the input of themultiplexer 104 is closed initially upon receipt of a triggering pulseon an associated conductor, for example, the conductor 32A until theultrasonic oscillations generated by the trigger pulse fade away,whereupon first the associated input of the multiplexer is opened inorder to detect groups of reference and measuring oscillations appearingat the output of the connected converter on the conductor (in theexample conductor 32A) for processing. Connected to the output of themultiplexer 104 is a high pass filter 105 which functions as apreamplifier with a second order high pass suppression and which has anoutput connected to the comparator 108. This comparator as described infurther detail below serves to determine the zero crossing point markingthe end of the actual reference travel time in a group of referenceoscillations and for determining zero crossing point at the end of thetravel time for the group of measurement oscillations.

The output signal of the high pass filter is directed either to theinput of a comparator 108 or to an amplifier 110, the amplification ofwhich is controlled by the controller 84, and a probability detector 112is connected to the output of the amplifier 110. The signal input to theprobability detector arrives at the input of a multiplier 114 and theinput of a delay circuit 116, the output of which is the second input ofthe multiplier 114. The multiplier 114 and the delay circuit 116essentially form a rectifying circuit, and in this embodiment a squarecircuit. Connected with the multiplier 114 is an integrator 118 servingas a zero error integrator. With a zero error integrator the outputsignal corresponds essentially to the integration of a variable inputmagnitude, and in the case of an input magnitude of null value, theoutput in general again assumes a null value with a relatively largetime constant. Connected to the output of the integrator 118 arethreshold detectors or switches 120, 122.

The operation of the detection circuit 34 (FIG. 1), is described belowfor the embodiment of FIG. 3 with the aid of FIG. 4. Accordingly, thetriggering of the converter connected to the conductor 32A and thedetection of its output signal are illustrated; triggering and detectionof other converters connected to the conductors 32B to 32D are producedin a corresponding manner.

At the timing point t_(o), the control register 106 causes the pulsegenerator 94 to start and the output of the generator is illustrated ascurve A in the uppermost portion of FIG. 4. The impulse generator 94produces two square pulses i₁ and i₃ with a period t₁ and with a pulseduration equal to half of the period.

The signals on the conductor 32A are illustrated as curves B in thesecond portion of FIG. 4. The pulses i₁, i₃ produced by the pulsegenerator 94 operate as trigger pulses for the group of transmittedoscillations with half waves I₁, I₂, I₃ . . . , since the conductance ofthe coil 100A together with the capacitance of the oscillator 48 (FIG.2) in the converter 30 connected to the conductor 32A forms a resonantcircuit excited by the impulses i₁, i₃. The period T₁ of the pulses i₁,i₃ is selected so that the second square wave pulse i₃ begins just asthe second half wave I₂ of the group of transmitted oscillations S hasreached its full amplitude. The group of transmitted oscillations S areprevented from reaching the switching components connected to themultiplexer 104 by the multiplexer itself; the multiplexer 104 joins itsinput connected to the conductor 32A initially with its own output whenthe group S of transmitted oscillations fades to zero, namely after apredetermined time period T₂ from the starting time t₀. Accordingly, theoscillations occurring in the output signal of the converter connectedto the conductor 32A are processed through the multiplexer 104.

As shown in curve B, a group R of reference oscillations with half wavesI_(1R) I_(2R), I_(3R) . . . appears in the output signal due to thereception of reflected ultrasonic oscillations from the referencereflection surface 66 (FIG. 2). Later, the group M of measurementoscillations with half waves I_(1M), I_(2M), I_(3M) . . . appears in theoutput signal due to the reflection region at the second end wall 28(FIG. 1).

The actual reference travel time is that travel time that is necessaryfor a particular transmitted half wave to again travel back from theconverter 30 after reflection at the reference reflection surface 66(FIG. 2). Thus, the actual reference travel time can be viewed, forexample, as the time period between the zero crossing point between thesecond half wave I₂ and the third half wave I₃ of the transmitted groupS of oscillations to the null crossing point between the second halfwave I_(2R) and the third half wave I_(3R) of the group R of referenceoscillations. In addition, the mentioned zero crossing point in thetransmitted group S of oscillations can mark the beginning of the actualtravel time and the mentioned zero crossing point in the group R ofreference oscillations can mark the end of the actual travel time to bemeasured. Therefore, in the exemplary embodiment, the actual beginningof the travel time to be measured is not marked, but only the end of thetravel time is marked in relation to the starting point t₀. Further, theprocessor 38 calculates a provisional value of travel time from whichthe actual travel time is determined through subtraction of a constant,since, for example, the zero crossing point between the second half waveI₂ and the third half wave I₃ of the transmitted group S of oscillationshas a predetermined time position with respect to the starting point t₀.This type of travel time measurement with an arbitrarily markedbeginning of the travel time is indeed valid for the measurement of theactual reference travel time T_(R) as well as the travel time of themeasurement oscillations T (FIG. 4 below).

The detection of the group R of reference oscillations and the group Mof measurement oscillations results from the same elements of thedetection circuit 34 in the exemplary embodiment of the invention. Inparticular, the marking of the end of the actual travel time to bedetermined, namely, the actual reference travel time T_(R) or the traveltime T of the measurement oscillations, is accomplished with the aid ofthe zero crossing of the trailing portion of a half wave and, to besure, in the exemplary embodiment, the second half wave I_(2R) of thegroup R of reference oscillations and the second half wave I_(2M) of themeasurement oscillations with the cooperation of the comparator 108which represents a first threshold detector.

The output signal of the comparator 108 is illustrated in the fourthportion of FIG. 4 as curve D. In the crosshatched region, the outputsignal is not defined, nor is detection of any interest. In the regionof the group R of reference oscillations and the group M of measurementoscillations, the output signal D varies or changes its logic condition(Low to High) at a zero crossing point of the group R of referenceoscillations and correspondingly the group M of measurementoscillations.

The output signal D by itself would still not provide any information ofwhen the end of the travel time T, T_(R) to be measured should bemarked, since it displays a plurality of changing conditions. One choiceof these varying conditions makes it feasible for the comparator 108 tooperate in a dynamic fashion, for a change in condition of the signal Dfrom High to Low to take place with the changing of a positive halfwave, for example, I_(1R) to a negative half wave, in the exampleI_(2R), and for a change in condition of the signal D from Low to Highto take place with a change of a negative half wave to a positive halfwave, for example, from I_(2R) to I_(3R). Thus, the approaching zerocrossing point of the second pulse I_(2R) or correspondingly I_(2M) forthe marking of the travel times T_(R) or correspondingly T lies in theleading portion of the pulse appearing in the signal D. Therefore,additional means must be provided for establishing which of the leadingportions occurring in the signal D is approaching for marking of thetravel time. This comes about essentially through means for detectingthe existence of the group R of reference oscillations and the group Mof measurement oscillations in the output signal of the converterconnected to the conductor 32A; if the presence of either the group R ofreference oscillations or the group M of measurement oscillations isdetected, then it can be stated or declared after the production of acorresponding signal, if it follows directly thereafter, that the secondfrom the beginning of the signal or the third from the beginning or someother change in the signal D designates the zero crossing point beingsought.

As a means for detecting the presence of the group R of referenceoscillations or group M of measurement oscillations, it is basicallysufficient to provide a second threshold circuit in parallel with thefirst threshold circuit--formed in the exemplary embodiment by thecomparator 108 with a finite switching level which transmits an outputsignal and only if the output signal of the monitored converter exceedsa particular amplitude. The output signals of both threshold circuitscan then be so interconnected that they together form a hysteresisamplifier of which the output signal then indicates only a particularchange in condition (for example, the leading edge of a pulse) if anegative half wave exceeds a sufficient amplitude in a positive halfwave. By this means, it can be established with relatively highcertainty that the zero crossing point at the end of a second half waveI_(2R) or correspondingly the zero crossing point at the end of thesecond half wave I_(2M) with the measurement of the measurementoscillation travel time T. In the exemplary embodiment, another solutionis actually selected in contrast to these other possible solutions, andalthough the selected solution requires a greater structural expensebecause of the presence of the amplifier 110 and the detector 112;nevertheless, it offers a particularly high assurance of an exactdetermination of the sought after zero crossing point.

In the third portion of FIG. 4, the amplification factor of thecontrollable amplifier 110 relative to its application is illustrated incurve C. During the time period T₂ in which the transmitted group S ofoscillations is produced and fades away again, the amplification factorcan basically be selected at any desired value since the multiplexer 104does no permit the transmitted group S to pass through to the amplifier110. Nevertheless, the degree of amplification during this period oftime is purposely adjusted to avoid a drift at the null condition.During the time period T₃ following this, the degree of amplification isadjusted to a relatively high, constant value. The time period T₃ formsthe window in which the group R of reference oscillations occurs in eachcase when different actual reference travel times T_(R) are produced dueto various temperatures, pressure, and other conditions of the fluidmedium in the cylinder chamber 26 (FIG. 1). The group R of referenceoscillations is therefore amplified at a relatively high amplificationfactor in the amplifier 110. During the adjacent time period T₄ whichterminates in a manner not illustrated prior to the starting point t₀ ofthe subsequent measurement, the amplification factor of the amplifier110 increases upwardly in a manner corresponding to an exponentialfunction. Bear in mind from this that the amplitudes of the group ofmeasurement oscillations in the output signal of one converter decreaseexponentially with the relative distance in corresponding travel time tobe measured due to the housing in the fluid medium of the cylinderchamber 26 (FIG. 1), so that a compensation results in a manner suchthat in the output signal of the amplifier 110, the amplitudes of thegroup M of measurement oscillations are not dependent upon the fact thatthe actual travel time of the oscillations remains constant. The constntamplification factor during the period T₃ is selected so that theamplitudes of the group R of reference oscillations amplified therebyare approximately equal to the corresponding amplitudes of the group Mof measurement oscillations amplified in the previously describedmanner, and so that therefore at the output of the amplifier 110, forexample, the amplitudes of the second half wave I_(2R) of the group R ofreference oscillations equal the amplitudes of the second half waveI_(2M) of the group M of measurement oscillations, the amplified thirdhalf wave I_(3R) has the same amplitude as the amplified amplitude ofthe third half wave I_(3M), and so on.

The output signal of the multiplier 114 is illustrated as curve E in thefifth part of FIG. 4. Contrary to the alternating voltage of the abovedescribed reference and measurement oscillations in the output signal ofthe converter connected to the conductor 32A, the output signal of themultiplier is rectified and for this purpose is squared in the preferredembodiment due to the operation of the multiplier 114. The integratedresult obtained at the output of the integrator 118 due to theintegration of the signal is illustrated in the sixth portion of FIG. 4as curve F, the two portions F_(R) and F_(M) corresponding respectivelyto the group R of reference oscillations and the group M of measurementoscillations.

The integrated results illustrated by the curve F and in particular theparts F_(R) and F_(M) provide an indication of the probability that anoscillation occurring on the conductor 32A is a group of referenceoscillations or correspondingly a group of measurement oscillations, andthat these groups were received without distortion and therefore aredetectable. So, for example, reflections within the cyliner 26 (FIG. 1)can lead to a distortion or even a phase discontinuity in the group M ofmeasurement oscillations whereby the amplitude of the part F_(M) isessentially reduced. From experience, the first two half waves I_(1M),I_(2M) are at least affected by such distortions or phasediscontinuities, and this establishes a reason for detecting the nullcrossing at the end of the second half wave I_(2M), whereby a preferenceis established for the larger amplitude of the second half wave over thezero crossing point at the end of the first half wave.

The output signal of the threshold signals 120, 122 are illustrated ascurves G, K in the two lower portions of FIG. 4. The threshold circuit120 is triggered without hysteresis at a relatively low value of theintegration value corresponding to a probability w₀. The thresholdcircuit 122 is triggered at a higher value of the integration resultcorresponding to a higher probability w₁, but this value is actuallynotably lower than the maximum values of the parts F_(R), F_(M). Thesmaller probability w₀ is reached at a point in time t₁ orcorrespondingly t₃ shortly after the beginning of the group R ofreference oscillations or the group M of measurement oscillationsrespectively. The higher probability w₁ is exceeded at somewhat latertimes t₂ or t₄.

It has been shown that with a suitable choice of the probability w₀, thezero crossing point of the trailing edge of the second half wave I_(1R)or correspondingly I_(2M) always has a uniform timing position relativeto the application of the signal to the threshold circuit 120 at timingpoints t₁ or t₃ respectively; with the selection of the probability w₀set out in FIG. 4, the null crossing point being sought is that onewhich takes place as the first null crossing point after the times t₁,t₃. It is apparent that with selection of a higher value for theprobability as the trigger point for the threshold circuit 120, forexample, with an order of magnitude of a probability w₁ selected in thepreferred embodiment, the null crossing point being sought can also bethat one which lies timewise directly before the application of thesignal to the threshold circuit 120 and even with selection of adetected null crossing point, the correspondingly detected change ofcondition in the signal D can also be the one after the timing point t₁or t₃. In each case, however, with several successive changes in thecondition in the output signal from the comparator 108 orcorrespondingly some other first threshold circuit, only that change incondition is effective for marking the end of the travel time T_(R) orcorresponding T which has a predetermined time position relative to theoutput signal of the threshold circuit 120. This time position can beestablished in a simple manner by means of suitable logic coupling meanswhich can be provided at the input of the memory 86 or alternately inthe processor 38.

The time interval t₂ -t₁ or correspondingly t₄ -t₃ which the thresholdcircuit 122 and the threshold circuit 120 correlate with the increase inprobability at the beginning of the portions F_(R) or correspondinglyF_(M) is a measure of how well the group R of reference oscillations orcorrespondingly the group M of measurement oscillations would bereceived without disturbance or distortion at least in regard to the twofirst half waves. If the time interval t₄ -t₃ is extended from anundisturbed condition, then it would lie at a distortion or damping ofthe group M of measurement oscillations whereby its detection would bemade more difficult. In order to again establish detection of the timeinterval, the degree of amplification of the amplifier 110 is increasedin this case, until the time interval t₄ -t₃ again has it originalvalue. Thus results a regulation of the degree of amplification of thecontrollable amplifier 110 during the time period t₄ in the sense of astandardization of time interval t₄ -t₃. In a corresponding manner,regulation of the degree of amplification occurs in the time interval T₃in the sense of a standardization of the interval t.sub. 2 -t₁, in orderto guarantee the detectability of the group R of reference oscillations.

The regulation as described above is produced by suitable means in theprocessor 38. Fundamentally, it would be possible to set the degree ofamplification of the amplifier 110 for each individual travel timemeasured on the basis of a few previously measured values of the timeintervals t₄ -t₃ and t₂ -t₁. In the preferred embodiment, nevertheless,the basic time period is established qualitatively, as is illustrated inthe curve C, by means of a suitable program control in the control unit84, for example, by means of a subroutine program memory 124 in thecontrol register 106. This has the advantage that the processor 38 onlyhas to produce an adjustment of the degree of amplification with thelarger time constants or, with the large opposing distances relative tothe frequency of the travel time measurements, in which case, forexample, the amplification values stored in the subroutines aremultiplied with a suitable correction factor.

In the preferred embodiment, the operating time of the delay device 116is selected such that the delay at the frequency of transmittedoscillations S equal to 500 kHz corresponds to a phase shift of a halfwave length. By this means, the squaring circuit formed from themultiplier 114 and the delay member 116 immediately becomes a selectivefilter at the frequency of the transmitted oscillations. Thiscontributes to the fact that shortly after the beginning of a group R ofreference oscillations or a group M of measuring oscillations, theintegrated result F provides an indication of whether these oscillationsare not distorted in regard to frequency and phase relationship.Basically, however, it would also be possible to replace the multiplier114 and the delay member 116 with a rectifier. Also, other variations incontrast to the described embodiment are understandably possible in thelight of the patent claims.

We claim:
 1. A method for determining the distance between a cylinder (12) and a piston (14) in a piston and cylinder assembly (10), which has a cylinder chamber (26) filled with a fluid medium and including end walls formed by the cylinder (12) and piston (14) comprising the steps of:transmitting a group (S) of sequential ultrasonic oscillations from a first end wall (24) of the cylinder chamber (26) toward the second end wall (28) for reflection in the vicinity of the second end wall as a group (M) of measurement oscillations, the step of transmitting being performed by an ultrasonic converter supported at the first end wall (24) and including an incompressible carrier (44) having a flat surface (46), a plate-shaped piezoelectric oscillator (48) positioned parallel to the flat surface (46) and, in spaced relationship from the surface, a sound projecting surface (42) positioned parallel to the oscillator (48) and facing the second end wall, and means (50, 52, 54) for mismatching the characteristic impedance of the oscillator (48) and the carrier (44), and means (56, 58, 60, 62) for matching the impedance between the oscillator and the sound projecting surface (42); receiving the group (M) of measurement oscillations at the first wall after reflection in the vicinity of the oppositely disposed second wall for measurement of the elapsed travel time of the oscillations between transmitting of the group (S) of ultrasonic oscillations and the receipt of the group (M) of measurement oscillations to establish a measure of the relative distance; establishing a reference path (42, 66) of a prescribed length within the fluid medium of the cylinder chamber (26) with a reflection surface (66) at one end of the path; transmitting a group of ultrasonic oscillations along the reference path from a first end for reflection at the surface (66) at the opposite second end; receiving the oscillations reflected from the reference reflection surface (66) as a group (R) of reference oscillations at the first end; measuring the elapsed travel time (T_(R)) between the transmission and receipt of the ultrasonic oscillations along the reference path at the first end (42) and producing a reference signal proportional to the elapsed time; and establishing the relative distance between the cylinder and piston by modifying the measurement of the elapsed time of the group (M) of measurement oscillations in accordance with the reference signal as compensation for the influence of different propagation conditions within the cylinder chamber.
 2. A method according to claim 1 wherein the steps of transmitting the group (S) of ultrasonic oscillations from the first end wall toward the second end wall for reflection of group (M) of measurement oscillations and detection of the actual travel time (T) of the measurement oscillations and the step of transmitting the ultrasonic oscillations along the reference path for reflection at the surface (66) and detection of a reference travel time are performed simultaneously so that a group (R) of reference oscillations and a group (M) of measurement oscillations are received in the region of the first end wall (24) one after the other.
 3. A method according to claim 1 wherein the steps of transmitting the group (S) of ultrasonic oscillations and receiving the group (M) of measurement oscillations are both performed by the ultrasonic converter.
 4. A method according to claim 3 wherein the step of receiving the group (R) of reference oscillations is also performed by the ultrasonic converter.
 5. An apparatus for determining the relative distance between a cylinder (12) and a piston (14) of a piston and cylinder assembly (10) that defines a cylinder chamber (26) filled with a fluid medium, the end walls (24, 28) of the cylinder being formed by the cylinder head (12) and the piston (14) comprising:electrical ultrasonic converter means (30) supported at the first end wall (24) of the cylinder chamber (26) for transmitting a group (S) of ultrasonic oscillations in the direction of the second wall (28) of the cylinder chamber (26) upon excitation by means of electrical excitation pulses (i₁, i₃), and also producing a corresponding electrical output signal upon impingement of ultrasonic oscillations, the converter including an incompressible carrier (44) having a flat surface (46), a plate-shaped piezoelectric oscillator (48) positioned parallel to the flat surface (46) of the carrier (44), and positioned in the cylinder chamber (26) in spaced relationship from the surface (46), a sound projecting surface (42) positioned parallel to the oscillator (48) and facing toward the second end wall (28) that is in contact with the fluid medium, the projecting surface preferably being formed from the same outer surface as one side of the oscillator (48), and means (50, 52, 54) for mismatching the characteristic impedance of the oscillator (48) and the carrier (44), and means (56, 58, 60, 62) for matching the impedance between the oscillator and the sound projecting surface (42), electrical pulse generating means (94) coupled with the output of the converter means (30), means defining a reference path (42, 66) extending through the fluid medium that is employed in the cylinder chamber (26), and including at a second end opposite the first end a reference wave reflecting surface (66) for reflecting ultrasonic oscillations; means for transmitting a group (S) of ultrasonic oscillations from the one end of the reference path toward the reflecting surface at the second end; and detection circuit means (34) including means for measuring the travel time of the oscillations through the cylinder chamber between the leading edge of an excitation pulse (i₁, i₃) and the occurrence of a corresponding group (M) of measurement oscillations in an output signal of the converting means, and also for measuring the actual reference travel time (T_(R)) of the ultrasonic oscillations transmitted and reflected along the reference path (42, 66), and further including calculating means (38) for modifying the measurement oscillation travel time in accordance with the departure of the actual reference travel time (T_(R)) from a pre-established reference travel time to establish the true relative distance between the piston and cylinder.
 6. Apparatus according to claim 5 wherein the sound reflecting surface (42) of the converter is a flat sound reflecting surface (42) associated with one of the two end walls (24) in contact with the fluid medium, and the reflecting surface (66) of the reference path is positioned at a pre-established minimal distance from the converter (30) in comparison to the variable distance of the second end wall (28) from the first end wall (24), the reference reflecting surface having a perpendicular deviating by a minimal amount from the perpendicular of the sound reflecting surface (42) of the converter, and being smaller than the sound reflecting surface (42) so that a group (R) of reference oscillations and thereafter a group (M) of measurement oscillations are received by the converter after a group (S) of ultrasonic oscillations are transmitted simultaneously along the reference path and through the cylinder chamber from the converter, the groups appearing in an output signal of the converter sequentially in timed relationship.
 7. An apparatus according to claim 6 wherein the converter produces at its output a filtered, amplified, and amplitude regulated signal; and wherein a first threshold circuit (108) is connected to the converter to receive the output signal, and has a trigger point set at the zero crossing point of its input signal; means are provided for detecting the presence of the group (R) of reference oscillations and the group (M) of measurement oscillations in the output signal of the converter (30) for measuring the actual reference travel time (T_(R)) and the travel time (T) of measurement oscillations to determine the end of the travel times (T_(R), T) in response to the changing condition of the output signal (D) of the first threshold circuit means (108) in response to the detection of the groups (R, M) of reference and measurement oscillations respectively.
 8. Apparatus according to claim 7 characterized in that the means 112 for detecting the presence of the group (R) of reference oscillations in the group (M) of measurement oscillations in the output signal of the converter includes circuit means (114, 116) for rectifying and preferably squaring the output signal of the converter, an integrator (118), preferably a zero error integrator, connected with the output of the circuit means (116), and a second threshold circuit (120) connected with the output of the integrator and providing an output signal (G) when the results of the integration (F) exceed a pre-established threshold (w₀), and logic coupling means connected with the first threshold circuit (108) for rendering effective only those changes in condition of the output signal utilized for marking the end of the travel times (T_(R), T) after several successive changes in condition of the output signal (D) from the first threshold circuit (108) which changes have a predetermined time relationship with respect to the output signal (G) of the second threshold circuit, preferably the changes in condition occurring with the zero crossing at the end of the trailing edge of the second half wave (I_(2R), I_(2M)) of the groups (R, M) of reference and measurement oscillations respectively.
 9. Apparatus according to claim 8 further including an amplifying means (110) disposed in a signal transmission path (102, 110, 112) between the converter (30) and the second threshold circuit (120) and interconnected preferably with the circuit (114, 116) for rectifying and having a controlled amplification, a third threshold circuit (122) with an input connected in parallel with the input of the second threshold circuit (120) and providing an output signal (K) when a relatively higher second value (w₁) of the integration results (F) exceeds the threshold value (w₀) of the second threshold circuit (120), means (38) for measuring the time durations (t₂ -t₁ ; t₄ -t₃) between the output signals (G, K) of the second and third threshold circuits (120, 122) and means (38, 124) for regulating the degree of amplification of the controlled amplifier (110) as stabilization of said durations.
 10. Apparatus according to claim 9 further including program transmitter means (124) connected to the controllable amplifier (112) and having a stored program holding the amplification at a constant value during a time period (T₃) comprehending the occurrence of the group (R) of reference ocillations, and subsequently increasing the amplification progressively from a small value corresponding to the constant value in an exponential manner, and means for controlling the degree of amplification to cause the program transmitting means (124) to modify the output signal controlling the amplification.
 11. Apparatus according to claim 5 wherein the means for matching impedance includes at least a quarter wave plate (50, 54) having a wave impedance several times smaller than the wave impedance of the oscillator (48) and preferably composed of two such plates (50, 54) with an interposed quarter wave plate (52) having a substantially higher characteristic impedance and the means for matching the characteristic impedance comprises a plurality of quarter wave plates (56, 58, 60, 62) having characteristic impedances lying between that of the oscillator and the fluid medium and decreasing in a stepped manner with one of the plates (58) lying at or near the beam projecting surface (42) being formed from a material suitable for manufacturing circuit boards and fixed means (30) for providing a reference reflection surface (66). 